This invention relates to rotors used in electrical machines, and more particularly, this invention is related to controlling the torsional natural frequency of a rotor.
Rotors are used in many different types of electrical machines, including generators, motors and other similar devices. The rotor in large cylindrical rotor generators is typically made of a single magnetic steel forging. The rotor is cylindrically configured with an outer peripheral surface, having winding slots that are defined by radially projecting winding teeth positioned on the rotor body. Winding wire is then looped about the winding slots at specific areas to form a winding coil.
The rotor winding slot has a north and south pole and a direct (D) axis aligned with the poles and a quadrature (Q) axis midway between poles. The rotor has a natural torsional frequency that is affected by several parameters, mainly, its torsional stiffness and added polar mass moment of inertia WR2.
For reasons of stress, fatigue, performance and other similar reasons, it is desirable to have the torsional natural frequency of the rotor avoid certain values or a range of values. There are numerous prior art methods and systems presently available to control or change the torsional natural frequency of a rotor, especially by the selective choice or changes to any shaft and rotor geometry. Each of these tuning methods and systems, such as the use of geometry choices or changes, typically exhibit a negative effect on another parameter, such as the cost, size, performance, and ability to manufacture the overall machine design.
Other rotor geometries can be designed to suppress undesired pulls and field distortion, production of noise, reduce bearing wear, and increase the power output of a motor. For example, in U.S. Pat. No. 2,698,910 to Blyth, an armature has a toothed structure to form an asymmetrical relation to the motor field and provide a means for suppressing the undesired pulls and field distortion production of noise. It also reduces bearing wear. A plurality of small, half-round dummy slots, or grooves, are positioned in the center of each armature tooth.
U.S. Pat. No. 4,827,172 to Kobayashi discloses a DC motor rotor core having a plurality of shallow and deep slots that are circumferentially and alternatively disposed in the peripheral face of the rotor core at angular intervals about the center axis. This structure increases the power output of the motor by increasing the number of turns, but does not work for torsional tuning by adjustment and selection in a desirable manner.
The present invention is advantageous and now provides torsional tuning with less negative impact on the overall machine design than prior art methods. In accordance with one aspect of the present invention, a method for tuning the torsional natural frequency of a rotor is disclosed and comprises the step of forming within winding slots defined by radially projecting winding teeth at least one tuning slot that extends radially inward from the bottom of the winding slot a distance to tune the rotor to a desired torsional natural frequency. This tuning slot can have a width smaller than the width of any winding wire received within the winding slot to prevent winding wire from passing into the tuning slot. The tuning slot can be positioned at a location that minimizes impact to the electromagnetic characteristics of the rotor cross-section, such as in slots in proximity of the Q-axis. At least one tuning slot that extends radially inward from the bottom of the winding slot is positioned at the quadrature axis.
In yet another aspect of the present invention, a rotor includes a rotor shaft and a cylindrically configured rotor body having a plurality of radially projecting winding teeth that define winding slots for receiving winding wire therein. The winding slots have a bottom that is spaced radially inward. At least one tuning slot extends radially inward from the bottom of the winding slot a distance that tunes the rotor to a desired torsional natural frequency.